Processes for synthesizing dianhydrohexitol di(alkyl carbonate)s have been described in the patent application US 2004/241553 and in the Japanese document JP 6-261774 by reaction between a dianhydrohexitol and a chloroformate ester. However, a major disadvantage of such processes is the use of toxic compounds such as the chloroformate esters.
A safer and environmental benign transesterification process avoiding toxic reactants is described in the patent application US 2012/0041169 in which a 1,4:3,6-dianhydrohexitol, particularly isosorbide, is reacted with a dialkyl carbonate, preferably dimethyl carbonate (DMC) and diethyl carbonate (DEC), in the presence of a transesterification catalyst, such as a base, preferably potassium carbonate (K2CO3) lithium hydroxide (LiOH) and potassium hydroxide (KOH) and to give the corresponding 1,4:3,6-dianhydrohexitol di(alkyl carbonate) in good yields shifting the equilibrium to right by distilling continuously the alcohol or the azeotrope formed. However, a drawback of this process is that a high fraction of oligomers (oligocarbonates) is formed as by-products, at the expense of the desired 1,4:3,6-dianhydrohexitol di(alkyl carbonate), in particular when a relatively low dialkyl carbonate/dianhydrohexitol molar ratio is used, for example of less than or equal to 20 or 10. According to US 2012/0041169, in order to limit the oligomer fraction to approximately 5% of the solvent-free reaction mixture, a molar excess of dialkyl carbonate greater than 40 must be used which leads to a lower productivity and, hence, to an undesirable increase in the production plant size. Furthermore, the greater the excess of dialkyl carbonate used the more energy will be consumed to remove this chemical from the reaction mixture by evaporation at the end of the transesterification reaction. Moreover, to obtain a 99% pure 1,4:3,6-dianhydrohexitol di(alkyl carbonate) a costly purification step either by distillation under high vacuum (<1 mbar) or by crystallization is employed. Likewise, another drawback of this process is the high amount of catalyst used, comprised between 0.1 and 10 molar equivalents which is very particularly between 1 and 3 molar equivalents with respect to the amount of dianhydrohexitol.
In view of the above, there is still the need in the state of the art of providing an improved process for manufacturing 1,4:3,6-dianhydrohexitol di(alkyl carbonate)s which overcomes at least part of these drawbacks.